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The Daily Q: How did the Census estimate population growth?

New York City is growing faster than expected. The Census Bureau estimates that the city’s population grew by 69,777 since April 1, 2011, an increase attributed to higher birth rates and immigration. While the director of the New York City Department of City Planning’s Population Division, Joseph Salvo, said he welcomed the news, he added that the feds…

New York City is growing faster than expected. The Census Bureau estimates that the city’s population grew by 69,777 since April 1, 2011, an increase attributed to higher birth rates and immigration. While the director of the New York City Department of City Planning’s Population Division, Joseph Salvo, said he welcomed the news, he added that the feds routinely undercount New Yorkers. The bureau says the city’s total population is about 8.2 million but the city claims the real number is 8.4 million. Recently Salvo went as far to officially challenge the bureau’s numbers but was unsuccessful.

That got us at the New York World asking: how are new estimates calculated?

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What we found

Every year the Census Bureau prepares population estimates for all counties in the United States. The agency assumes that after the decennial census the population isn’t going to alter drastically, so any changes can be closely approximated through other statistics.

The most important numbers demographers look at are birth rates, death rates and net domestic migration, meaning the number of persons entering or leaving from other counties. All of these numbers are relatively easily to calculate and reliable.

Where things start getting fuzzy is with net international migration, which includes immigration of foreign-born and emigration of foreign and native-born people. The Census relies on the American Community Survey to estimate these numbers, but it’s far from perfect. Foreign-born individuals who indicate that they lived abroad in the prior year, for instance, are considered immigrants even if they’ve lived in the United States for years previously. In the latest estimate, one of the drivers of New York City’s population increase was immigration.

“It’s very hard to estimate the number of people who leave. Right now I think that’s the biggest problem we have,” said Andrew A. Beveridge, a sociologist at Queens College. “I don’t see a single data collection system that would handle that.”